1. Field of the Invention
This invention relates to a composition for use in enhanced oil recovery, i.e., for recovery hydrocarbons from subterranean reservoirs, and to a method of carrying out such a process. More particularly, the invention relates to such a composition and method utilizing a polymer-thickened fluid having improved hydrocarbon-displacement properties.
2. Description of the Prior Art
In enhanced oil recovery processes, it is known to inject into a reservoir, via one or more injection wells, various displacement fluids which pass through the reservoir and assist in removing therefrom liquid hydrocarbons which are subsequently produced via one or more production wells. Oftentimes, the liquid hydrocarbons are relatively viscous compared to the displacement fluid. Thus, the injected displacement fluid has a tendency to finger through the liquid hydrocarbons in the reservoir, bypassing and leaving unrecovered a substantial quantity of the hydrocarbons. When the displacement fluid is an aqueous base fluid, it is known to add thereto a water-soluble or -dispersible polymer thickener to increase the viscosity of the displacement fluid to a value which more nearly matches the viscosity of the reservoir hydrocarbons. This thickened displacement fluid has less tendency to finger through the reservoir and is more effective in removing hydrocarbons therefrom. Particularly effective polymers of this type are the polyacrylamides of U.S. Pat. No. 3,002,960 to Kolodny and the partially hydrolyzed polyacrylamides of U.S. Pats. No. 2,827,964 to Sandiford et al., U.S. Pat. No. 3,039,529 to McKennon and U.S. Pat. No. 3,370,649 to Wolgemuth. In many instances, due to the lack of availability of fresh water or the nature of the reservoir, such as the presence therein of clays which swell in contact with fresh water and decrease reservoir permeability, or other reservoir materials which are not compatible with fresh water, it is desirable to use a natural or synthetic brine as the base displacement fluid rather than fresh water. In other instances, an injected fresh water base displacement fluid will contact and become mixed with reservoir brine as it passes through the reservoir. Many polymers, such as polyacrylamide and it derivatives, which impart a relatively high viscosity to fresh water, form solutions or suspensions having sharply lower viscosities if salts, such as those occurring in brine, are added to the system.
It is also known from the Sandiford et al. patent that optional additives, such as surface active or wetting agents, e.g., alkyl pyridinium salts, sorbitan mono-oleate, quaternary ammonium compounds and the like may be added to aqueous solutions of a water-soluble partially hydrolyzed polyacrylamide used as a displacement fluid in an oil recovery process. The surface active agents improve the contact between the aqueous flooding medium and the oil-wet particles of the reservoir.
A wide variety of surface active agents and surface active agent-containing solutions have been used in various aqueous base displacement fluids in enhanced oil recovery processes. One such material is "OK Liquid" marketed by The Procter and Gamble Company and described in U.S. Pat. No. 2,941,950 to Korpi et al. "OK Liquid" contains as one ingredient a sulfated and neutralized reaction product obtained by condensing 1 to 5 moles of ethylene oxide and 1 mole of a monohydric alcohol having from 10-16 carbon atoms in the molecule.
A somewhat similar surface active agent component is used in U.S. Pat. No. 3,500,923 to Reisberg which describes a waterflood process wherein the oil-bearing reservoir is penetrated by a slug of an aqueous solution containing a synergistic mixture of a water-soluble inorganic electrolyte such as an inorganic halide salt and a sulfated polyoxylated alcohol having from 8 to 30 carbon atoms or a salt thereof. Reservoirs conditioned by this pretreatment slug can subsequently or concurrently be subjected to a drive fluid. The drive fluid can contain the pretreatment composition plus thickeners, viscosity increasers, pushers and the like, e.g., polyacrylamides and derivatives thereof.
However, none of these prior art references deal with the problem of maintaining a high viscosity in aqueous solutions of polyacrylamide or polyacrylamide derivative which aqueous solutions contain or are contaminated by brine.
Accordingly, a principal object of this invention is to provide a composition and method for enhanced oil recovery wherein an aqueous displacement fluid containing polyacrylamide or a polyacrylamide derivative as a thickener retains a substantial degree of its viscosity in the presence of brine.
A further object of this invention is to provide such a composition and method for improving the efficiency of such a composition in the recovery of oil from a subterranean reservoir.
A still further object of this invention is to provide such a composition and method utilizing a brine-containing aqueous solution as the displacement fluid.
Another object of this invention is to provide such a composition and method useful in treating reservoirs containing brine.
Other objects, advantages and features will become apparent from the following detailed description and drawing.